As known heretofore, various structures and mechanisms display a resistance and resiliency to displacement or deformation. For example, the coil spring resists being stretched and upon release quickly returns to its original configuration. The relationship of the return force exerted by the spring to the displacement of the spring from its equilibrium position is known as the "stiffness" of the spring. The stiffness force exerted by the spring varies according to the displacement and is shown by the equation: F=kx; in which k is the stiffness of the spring and is a constant if the stiffness is linear, x is the displacement and F is the spring force or stiffness force.
For a simple mechanical system involving a coil spring having one end affixed to a stationary member and the opposite end connected to a displaceable mass m, the following formula is applicable: ##EQU1## where: k=stiffness of the spring
m=mass PA1 f.sub.n =resonant frequency
In the above mechanical system the stiffness force increases as the spring is moved to an extended posture from equilibrium. In the above formula the resonant frequency decreases as the stiffness value k is made to decrease.
To reduce the stiffness in a mechanical device such as a coil spring, mechanical force producing devices have been devised to provide a "negative stiffness", a term used to indicate that the force supplied is in a direction opposite to that of the normal stiffness force of the spring.
However, most such mechanical devices have numerous shortcomings and are difficult to construct, adjust, and incorporate for their beneficial features. Also, such mechanical devices have nonlinearity problems and oftentimes require relatively large areas for their application. Additionally, mechanical force producing devices can only correct the most simple stiffness problems and are therefore limited in their use.
Therefore, it is one of the objectives of this invention to provide an apparatus for reducing stiffness which is simple to construct and not overly expensive.
It is another objective of the present invention to provide an apparatus to reduce the stiffness in a mechanical system using electrodynamic means.
It is yet another objective of the present invention to provide an apparatus to reduce stiffness which is easy to adjust.
It is still another objective of the present invention to provide an electromechanical force-vs-displacement apparatus which will allow the user to achieve any desired force-vs-displacement curve, linear or nonlinear, over the entire operating range of displacement; including variation of a nonlinear stiffness to achieve a desired linear stiffness; and including creation of a totally negative stiffness device.